Inductively coupled plasma and microwave induced plasma optical emission spectroscopy (ICP-OES and MIP-OES respectively) are analytical techniques for determining the concentration of elements in a sample. A sample solution is injected into a plasma source, the temperature of which vaporises the sample, destroys any chemical bonds, ionises atoms and causes electronic excitation of the atoms and ions. A plasma spectrum consists of a continuum background and discrete spectral lines at wavelengths characteristic of any element contained in the sample. The intensity of any given spectral line is proportional to the concentration of the element in the sample and quantitative assessment of element concentration is then based upon a measurement of the intensity of light at a wavelength which is characteristic of the element.
A problem which exists in this and other analytical techniques involving emission spectra is that a continuum background can distort the shape of the spectral lines making an accurate assessment of element concentration difficult. Typical conventional techniques for providing some form of background correction are discussed in “ICP in Analytical Atomic Spectrometry”, Montaser & Golightly, VCH, 1987, section 6.6, and in “Median Filtering for Removal of Low-Frequency Background Drift”, Alvin W. Moore, James W. Jorgenson, Analytical Chemistry 1993, 65, 188-191. The simplest and a relatively unreliable technique is for the user to guess the wavelength value of one or two background points and use these wavelength values to apply a background correction in the determination of a concentration estimate. This initial choice of wavelength is typically used for all subsequent analyses of samples. Background correction is achieved at any particular wavelength of interest by linear interpolation between the background points. However this technique is time consuming and requires a relatively skilled technician to select suitable background points.
Similar problems exist in infra-red, gas chromatography, liquid chromatography and ultraviolet analytical instruments where changes in conditions affect the measurements made by the instrument.
Against this background the present invention has been made.